1. Field of the Invention
This invention pertains to polymer blends containing a polycarbonate (PC) and a block copolymer represented by the formula A-b-B (diblock); A-b-B-b-A (triblock); B-b-A-b-B (triblock); or (A-B).sub.n (multiblock) as examples, where A is a stereoregular polymethyl methacrylate block (s-PMMA) with at least 60% of its monomer units in the syndiotactic configuration and B is a second polymer block, such as polyisoprene (PiP), polybutadiene (PBD), polylauryl methacrylate (PLM) siloxane rubber or polybutyl acrylate (PBA); and "b" indicates a block structure. When thoroughly blended with polycarbonate, these new block copolymers enhance properties such as thick section toughness and low temperature impact strength.
2. Discussion of the Background
The term "thermodynamically miscible" is used in the art to define a polymer mixture that mixes on the molecular level to form a single, homogeneous phase which exhibits only one glass transition. The term "mechanically compatible" means that mixing of the polymers is on a small scale but larger than the molecular level. Furthermore, mechanical compatibility implies that the multiple phases exhibit good adhesion to one another and yield good mechanical properties. Although both thermodynamically miscible and mechanically compatible blends exhibit good mechanical properties, only thermodynamically miscible blends are transparent owing to their single phase nature.
Aromatic, polycarbonates generally have good thermal stability, good dimensional stability, good impact strength in thin sections, relatively good stiffness and most notably good transparency. For these reasons, PC is used in a variety of applications including glass replacement, housings, medical devices and containers. Nevertheless, PC does have drawbacks such as poor scratch resistance, poor long term U.V. stability and poor stress birefringence which must be dealt with, especially in demanding optical applications. Moreover, it is often desirable to also improve the processability, thick section toughness and low temperature impact strength of PC without sacrificing its transparency.
Methacrylate ester-based polymers suffer from poor dimensional stability and poor heat distortion yet have good clarity, surface hardness, U.V. resistance and processability. For this reason they are commonly used in applications such as window glazings, aircraft windows, automotive lenses and lightcovers. Thus, blends of PC and methacrylate polymers should have a good balance of properties and, if they formed a single phase, would be clear. Unfortunately ordinary (i.e., stereorandom) polymethylmethacrylate is not thermodynamically miscible with polycarbonate, meaning that their blends are opaque. This phenomenon has been reported in the literature many times and it has been often observed that such blends also suffer from poor low temperature impact strength and poor thick section toughness. The simple addition of a rubber impact modifier would greatly improve strength but would further reduce transparency because rubber impact modifiers are incompatible with both PC and polymethylmethacrylate. Thus, the task existed of finding methacrylic polymers that are thermodynamically miscible with polycarbonates and which could simultaneously be modified with a rubbery material like polyisoprene. The resulting blends would exhibit not only the improvements gained from blending PC and a methacrylic polymer, but would also have improved thick section toughness and low temperature impact strength, all while retaining the inherent transparency of polycarbonate.
Although blends of PC with methacrylic polymers are often mechanically compatible, resulting in improvements over the respective components, most are typically not miscible and so their opacity makes them unacceptable in many applications. The current understanding (Polymer, 32, page 272, 1991) is that traditional, free radically polymerized PMMA does not form a single, thermodynamically miscible, transparent blend with PC but does demonstrate mechanical compatibility. U.S. Pat. No. 4,319,003 teaches that blends of PC and polymethyl methacrylate (PMMA) are not only opaque, but often do not possess the advantageous properties expected of such a mixture. Among other references that report the immiscibility of PMMA with PC are JP 7216063 and EP 0297285.
Ways to overcome the immiscibility of typical PC/PMMA mixtures have, however, been disclosed. Most commonly employed is the addition of comonomers to the PMMA (DE 2264268; DE 3632946; and U.S. Pat. No. 4,906,696). In U.S. Pat. No. 4,319,003, the use of a PC-PMMA block copolymer is cited as an improvement over PC/PMMA blends themselves, but such materials cannot demonstrate clarity and mechanical property improvements simultaneously.
Similarly, mixing processes have been developed which can produce transparent PC/PMMA blends. According to DE 3,833,218, transparent mixtures of aromatic polycarbonates and polyalkyl methacrylates can be produced by melting the two components in the presence of a supercritical gas. Also U.S. Pat. No. 4,743,654 and U.S. Pat. No. 4,745,029 disclose that one may produce solutions of the two polymers in organic solvents, allow the solvent to evaporate, and thus produce a transparent film. Unfortunately all of these methods suffer from the drawback of bubble formation and other imperfections which would render them unsuitable for many applications. Since care is needed in such methods, these processes are slow relative to traditional melt forming processes like extrusion and molding. Furthermore, they are limited to small, thin parts like films owing to the need to remove gas and solvents. A further disadvantage of such processes is gradual deterioration and breakup of the material arising from phase separation of the two polymers since mixtures formed by these processes are metastable.
Recently, a number of patents and publications have appeared which report that PC is miscible with random copolymers containing methylmethacrylate and either cyclohexyl methacrylate or phenyl methacrylate. The thermodynamic miscibility of PC with pure polyphenyl methacrylate was also reported. EP 0297285; U.S. Pat. No. 4,906,696; J Appl. Polym. Sci. 44., 2233-2237, 1991; and Polymer 32(7) , 1274-1283 (1991).
I have recently discovered that stereoregular polymethyl methacrylate in which at least 60% of the monomer units are in the syndiotactic configuration (s-PMMA) is also thermodynamically miscible with polycarbonate in all ratios. That is, they form a single, homogeneous phase resulting in highly transparent and stable materials.
In the case of phenyl methacrylate, the miscibility property was used to increase the adhesion between a rubbery impact modifier, ethylene-propylene-diene (EPDM), and polycarbonate by grafting phenyl methacrylate onto EPDM. However, this graft copolymer was mechanically compatible with PC but not thermodynamically miscible, as might have been hoped.
U.S. Pat. No. 4,997,883 and EP 0326938 both teach the art of grafting aromatic(meth)acrylate/methyl methacrylate random copolymers onto a pre-existing EPDM polymer to prepare an elastomeric graft copolymer which, when added to PC, shows improvement in impact strength. Unfortunately, all of these materials are opaque. Thus, the task still existed to develop a means of blending PC with impact modifiers without simultaneous loss of clarity.
Block copolymers are a general class of materials which can exhibit a wide range of properties and are unique in their ability to "microphase separate" which refers to a fine separation of the two dissimilar polymer blocks into distinct phases.
It has now been discovered that block copolymers containing one or more blocks of s-PMMA (greater than 60% sydiotactic) are also miscible with polycarbonate, but since they microphase separate within the resulting blends, have the ability to disperse the second block effectively. The microphase of "second block" polymer is uniformly distributed throughout the polycarbonate and of a size smaller than the wavelength of visible light. This represents a distinct improvement over the approach where PC-miscible polymers are grafted onto an impact modifier.
The present invention also provides a synthetic route to the production of novel block copolymers containing s-PMMA from which mixtures of polycarbonate and s-PMMA block copolymers having greatly improved properties over the individual components, yet retaining the highly favorable characteristic of transparency can be prepared for the first time.
The present invention pertains to PC/(s-PMMA)-b-B binary blends, as well as ternary blends containing up to 15% of additional s-PMMA and/or B homopolymers. These compositions contain a thermodynamically miscible, single phase of PC and s-PMMA, as well as very finely dispersed particles of the "B" block which are usually on the order of 40 to 2,000 angstroms and thus much smaller than the wavelength of light. The combined effects of the single phase nature of the PC and s-PMMA, plus the very small size of the dispersed second block leads to an optically transparent material. Furthermore, the chemical attachment of the B block to the s-PMMA block assures perfect adhesion and translation of properties from the "B" block to the PC/s-PMMA phase. Thus, one can obtain a mixture which combines the advantageous properties of two dissimilar materials and still maintains transparency. Such a fine and stable dispersion can only be obtained through the use of block copolymers which have one block that is thermodynamically miscible with the PC. Therefore the basis of this invention arises from the coupling of: 1) the thermodynamic miscibility of the s-PMMA block and PC; 2) the phase separation behavior of block copolymers; and 3) the ability of the second block of the copolymer to impart beneficial changes to the PC matrix.
A general synthesis of well defined methacrylic ester containing block copolymers has only recently been accomplished (See for example: "Recent Advances in Mechanistic and Synthetic Aspects of Polymerization", Kluwer Academic Publishers, Norwell, Mass., 1987; and "Recent Advances in Anionic Polymerizations", Elsevier Publishing Co., New York, N.Y., 1987). These reports have focused primarily on polymers containing blocks of polymethyl methacrylate or polybutyl methacrylate made by an anionic polymerization mechanism. Anionic polymerization is used for the synthesis of well defined block copolymers because the reaction has no naturally occurring termination step. However, the presence of carbonyl groups initially caused problems with the polymerization of methacrylate monomers until methods were developed to prevent nucleophilic attack on the carbonyl groups. The most commonly accepted method combines the use of low temperature polymerization (-78.degree. C.) and modification of the initial anion, either by prereaction with 1,1-diphenylethylene, or by variation of its reactivity by reaction/chelation with pyridine and/or LiCl.